10 II. W. MARETT TIMS. 



an early age. lu the specimen No. 24 the bizygomatic breadth measured 64 mm., the 

 bicranial diameter being but 61 mm. It is doubtful whether the relations of these two 

 diameters is of any importance, since in the two skulls of Stenorrhynchus leptonyx 

 measured by Turner, the bizygomatic was greater in one, the bicranial in the other. 

 A point to which it is possible greater importance may be attached is that in the foetal 

 Leptonychotes the widest part of the zygomatic arch is at its posterior end, the breadth 

 gradually diminishing as one passes forwards. In this respect they agree with the adult- 

 skull of Stenorrhynchus, but differ from that of the adult Weddell's Seal, in which the 

 widest part is at the mid-point of the arch. The most interesting point which I have 

 observed is the extraordinary downward curve in the cervical region of the vertebral 

 column (PL II., fig. 12). The curvature involves the whole of the cervical and the 

 anterior portion of the dorsal region. The bend is so considerable that the ventral 

 surfaces of the vertebrae are brought so close to the ventral body wall that the 

 trachea and oesophagus are deflected to one side. Dr. Gadow made the suggestion 

 to me that it might possibly be a sexual character present only in the males and 

 caused by the habit of lifting the females when pairing. I therefore made median 

 sections of both sexes and found that the curvature is a constant feature, and further, 

 that it tends to become accentuated with the increasing age of the foetus. It is 

 evidently caused by the action of the powerful muscles on the dorsum of the neck 

 which, by approximating the head to the mid-dorsal region, have caused a " buckliug-up " 

 of the spinal column while in a cartilaginous and plastic condition. The particular 

 mechanical advantage to be derived by this condition is not quite easy to understand, 

 but apparently a short stunted neck is of value to aquatic animals as evidenced by 

 the Cetacea and Sireuia. In these mammals the shortening is brought about by an 

 antero-posterior compression and a partial fusion of the individual vertebral centra. 

 In the seals, however, the same end has* been attained by different means. I am not 

 aware of this fact having been noticed before ; it certainly is not shown by the 

 mounted skeletons which are to be seen in museums.* 



Muscular system. So detailed and careful a description of the muscles and their 

 attachments in Otaria and Trichechus having been given by Dr. Murie (9), it is 

 unnecessary for me to do more than note the points in which the muscles of these 

 animals appear to differ from those of the foetal specimens under consideration. 

 It is necessary, however, to repeat that the material was by no means in good 

 condition for dissection, the muscles being in a very brittle condition, so that, 

 in spite of care, the facts here recorded must be taken with a certain amount of 

 reservation. 



Writers on mammalian myology attach considerable importance to the muscles as 

 being of systematic and phylogenetic importance. Bearing this in mind, I have 

 compared the muscles of the embryo seal with the descriptions of the muscles of the 



* A mounted, but not exhibited, skeleton of Phoca vitulina in the Natural History Museum shows a 

 well-marked curvature in the cervical region. F. J. B. 





